Clock synchronization for a wireless communications system

ABSTRACT

Clock synchronization for a wireless communication system is described. The communication system utilizes a server with a radio coupled to receive a radio frequency (RF) signal and a clock interface to receive a reference clock signal. The server includes a network interface configured to receive, from a base station, a time that the RF signal was received at the base station. The server further includes a processing device configured to determine when the RF signal was transmitted and a location of the base station, and configured to calculate clock offset value representative of a time to delay a local clock signal at the base station to synchronize the local clock signal at the base station with the reference clock signal.

REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 11/804,156,filed May 15, 2007, which is hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates generally to wireless communications and inparticular but not exclusively, relates to synchronizing clocks in awireless communications system.

BACKGROUND

A high percentage of wireless communications is conducted with a mobiledevice located inside a building, while the wireless communicationsnetworks are conventionally implemented such that the cellular antennasare located outside of buildings. Inherent with in-building penetrationof cellular signals is an attenuation of the cellular signal strength.Thus, the attenuation of signal strength may result in a lack of, orreduced, communications coverage while the mobile device is locatedinside of a building. For example, the user of a mobile device may beunable to place or receive a telephone call while inside a house oroffice building.

One device used to compensate for weak cellular signals within abuilding is known as a home base station, also known as a femtocell.Home base stations are typically stand alone units deployed within abuilding, such as an office, a place of business, or even a home. Thesehome base stations provide two-way wireless voice and datacommunications coverage for a mobile device, thereby extending theeffective coverage area for the communications network. The home basestation supports cellular calls locally, and then uses a broadbandconnection to carry traffic to the communications network. One advantageof a home base station is that they typically operate with existingmobile devices rather than requiring users to use a separate phone oreven upgrade to an expensive dual-mode device.

Successful operation of a handoff of a mobile device between basestations depends on the stability and synchronization of the internalclocks of respective base stations. In particular, in order forrespective base station and mobile device transmissions to stay lockedto each other, the internal clock of the current base station must besynchronized with the internal clock of the base station to which themobile device is being handed off. In addition, synchronization of onebase station with a communications network reduces the likelihood thatcommunications between the base station and a mobile device willinterfere with other communications on the same network.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a functional block diagram illustrating a wirelesscommunications system, in accordance with an embodiment of theinvention.

FIG. 2 is a functional block diagram illustrating a base station, inaccordance with an embodiment of the invention.

FIG. 3 is a functional block diagram illustrating a network referenceserver, in accordance with an embodiment of the invention.

FIG. 4 is a flow chart illustrating a process for synchronizing a basestation to a wireless communications system, in accordance with anembodiment of the invention.

FIG. 5A is a flow chart illustrating a process for determining a clockoffset value, in accordance with an embodiment of the invention.

FIG. 5B is a flow chart illustrating another process for determining aclock offset value, in accordance with an embodiment of the invention

FIG. 6 is a timing diagram illustrating radio frequency signals and areference clock signal, in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

Embodiments of apparatuses and methods for clock synchronization for awireless communications system are described herein. In the followingdescription numerous specific details are set forth to provide athorough understanding of the embodiments. One skilled in the relevantart will recognize, however, that the techniques described herein can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

In a wireless communication network, one or more base stations areselectively positioned within respective geographic coverage areas orcells. These base stations are used to transmit and receivecommunication signals to and from mobile devices (e.g., mobile orcellular telephone handsets) located within a respective cell. Inparticular, the base stations act as intermediary points by which acommunication path may be periodically established and maintainedbetween mobile devices, as well as between a mobile device and an endpoint of a stationary network, such as, a landline connected to a publicswitched telephone network (“PSTN”).

There are a variety of a communication protocols in which mobile devicescan communicate (e.g., place and receive telephone calls) with a basestation of the communication network. For example, Code DivisionMultiple Access (CDMA), Time Division Multiple Access (TDMA), andUniversal Mobile Telecommunications System (UMTS) are all well knowncommunication protocols. Of concern with the selection of acommunication protocol is the ability of the mobile device tocommunicate with the base stations in a simple, flexible and rapidmanner so that the mobile device is not required to wait to establish acommunication path and that a hand off of an active call between basestations in a mobile network is transparent to a respective mobiledevice. In this respect, the ability to acquire and maintainsynchronization between base stations is an important consideration.That is, network-wide synchronization should be established andmaintained for optimal operation of a wireless communications network tominimize interference problems otherwise caused by non-synchronized basestations in adjacent cell locations.

For example, a TDMA protocol for a wireless communication networkincludes each base station transmitting over a set of time-division airchannels, or time slots. By transmitting during sequential time slots, abase station may communicate with a mobile device over an unoccupiedtime slot to establish a communications link. Each base station maythereby maintain communication with as many mobile devices as there areavailable time slots.

In accordance with this protocol, handoffs between base stations may beinitiated by the respective mobile device, which monitors available timeslots from the same and competing base stations during unused timeslots. A mobile device may then handoff to establish communication in anew time slot of the same base station, or may handoff in such a manneras to establish communication within a different base station.

Accordingly, embodiments of the invention are disclosed which providesynchronization of a base station in a wireless communications system.In one embodiment, the base station to be synchronized is a home basestation located in an office, home, or other wireless hotspot where thebase station is connected to a computer network via a broadbandconnection that includes network delays that are neither consistent norpredictable (e.g., cable and DSL).

Embodiments of the invention utilize radio frequency (RF) signalstransmitted from known locations, such as from broadcast televisiontowers or broadcast radio towers, to aide in the synchronization of thebase station's local clock. For example, in one embodiment, RF signalsare received at the base station and at a network reference server. Thenetwork reference server is also coupled to receive a reference clocksignal to which various base stations of the communications network areto be synchronized. A clock offset value is calculated, either at thebase station or at the network reference server, in response to thetiming of the RF signal received at the base station, the timing of theRF signal received at the network reference server, and in response tothe reference clock, itself. In one embodiment, the clock offset valuerepresents an amount of time to delay the local clock at the basestation such that it is synchronized with the reference clock of thecommunications network. The base station then synchronizes its localclock based on the calculated clock offset value.

FIG. 1 is a functional block diagram illustrating a wirelesscommunications system 100, in accordance with an embodiment of theinvention. The illustrated embodiment of wireless communications system100 includes a mobile device 102, a first base station 104, a secondbase station 106, radio frequency (RF) transmitters 108, a computernetwork 110, a network reference server 112, and a reference clock 114.

In the illustrated embodiment, wireless communications system 100provides wireless communications coverage for mobile devices, such asmobile device 102. In one embodiment, mobile device 102 is a portabletransceiver, such as a mobile or cellular phone. In another embodiment,mobile device 102 is a portable transceiver, such as a smart phone, apersonal digital assistant (PDA), a wireless pc card, a laptop computer,or the like.

First base station 104 and second base station 106 each provide wirelesscommunications coverage for mobile devices that are within a coveragearea of the respective base station. For example, when mobile device 102is within a coverage area 126, first base station 104 may providewireless communications coverage for the mobile device. Similarly, whenmobile device 102 is within a coverage area 128, second base station 106may provide wireless communications coverage for the mobile device. Inone embodiment, second base station 106 is a home base station wherecoverage area 128 extends substantially inside of a building, such as anoffice, place of business, or in a home. In another embodiment, firstbase station 104 includes a cellular tower located outdoors and is partof a cellular communications network.

To ensure continuity of communications, it may be desirable to havecoverage areas overlap, such as is shown by overlapped coverage area130. When a mobile device moves from one coverage area to another itgenerally switches communicating with a base station of one coveragearea to communicating with a base station of another coverage area. Thisprocess is known as handover or handoff. For example, mobile device 102may initiate communicating (e.g., placing a telephone call) throughfirst base station 104 while mobile device 102 is within coverage area126. As mobile device 102 moves from coverage area 126 to coverage area128 a decision is made that mobile device 102 should be handed off fromfirst base station 104 to second base station 106. This handoffprocedure generally occurs while mobile device 102 is within overlappedcoverage area 130 so that mobile device 102 can seamlessly transitionfrom communicating with first base station 104 to communicating withsecond base station 106. After the handoff is complete, mobile device102 may continue its communications, but now through second base station106.

In one embodiment, base stations 104 and 106 communicate with mobiledevice 102 via a Code Division Multiple Access (CDMA) standard, such asCDMA2000. In another embodiment, base stations 104 and 106 communicatewith mobile device 102 via a Time Division Multiple Access (TDMA)standard, such as Global Systems for Mobile communications (GSM). Instill another embodiment, base stations 104 and 106 communicate withmobile device 102 via a Universal Mobile Telecommunications System(UMTS) standard or a Worldwide Interoperability for Microwave Access(WiMAX) standard.

In the illustrated embodiment, reference clock 114 is coupled to providefirst base station 104 with a reference clock signal 118. In oneembodiment, reference clock 114 is a master reference clock used bycommunications system 100 to synchronize various base stations. Firstbase station 104 may be coupled to reference clock 114 via an ISDN(e.g., T1), a point to point microwave, or a fiber Optic (e.g., OC3)connection to receive reference clock signal 118. However, suchconnections may not be feasible for each base station in communicationssystem 100, due to cost limitations, or logistical constraints. Forexample, the illustrated embodiment of second base station 106 does notinclude a direct connection to reference clock 114. Instead, a localclock 116 at second base station 106 is synchronized with referenceclock signal 118 in response to one or more radio frequency (RF) signals124 transmitted by one or more RF transmitters 108.

In one embodiment, RF signals 124 include a synchronization signal suchas a periodic or cyclical timestamp. The timestamp provides a uniquereference upon which second base station 106 and NRS 112 may base theircalculations. In one example, RF signals 124 include broadcasttelevision signals (also known as over-the-air and terrestrialtelevision signals). Since, the location of RF transmitters 108 thatbroadcast television signals can be readily established, thesynchronization signals of broadcast television can be utilized withknown methods to determine a very accurate location of a device. Thesynchronization signal of broadcast television may include periodic orcyclical timestamp information such as the presentation time stamp (PTS)or decode time stamp (DTS) of several known television standards.

For example, RF signals 124 may include a National Television StandardsCommittee (NTSC) signal. In another example, RF signals 124 may includean American Television Standards Committee Digital Video Television(ASTC DTV) signal. In still another example, RF signals 124 may includea Digital Video Broadcasting (DVB) signal. In yet another example, RFsignals may include an Integrated Services Digital Broadcasting (ISDB)signal. In addition to, or as an alternative to broadcast televisionsignals, RF signals 124 may include broadcast radio signals, such as aFrequency Modulated (FM) radio signal, an Amplitude Modulated (AM) radiosignal, or a Digital Audio Broadcast (DAB) signal.

As is shown in FIG. 1, network reference server (NRS) 112 iscommunicatively coupled to second base station 106 via computer network110. Computer network 110 may be the public internet or alternativelymay be a private network. In one embodiment, second base station 106 iscoupled to computer network 110 via an IP broadband connection whichincludes network delays that are neither consistent nor guaranteed, suchas a Digital Subscriber Line or a cable modem connection. NRS 112 isalso coupled to receive reference clock signal 118 from reference clock114 via a communications link, such as an ISDN (e.g., T1), a point topoint microwave, or a fiber Optic (e.g., OC3) connection.

In operation, both second base station 106 and NRS 112 receive one ormore of RF signals 124. The RF signals received at second base station106 may then be used to calculate a physical location of the second basestation 106. In one embodiment, second base station 106 includes aprocessing device to calculate its location locally at second basestation 106. In another embodiment, second base station 106 transmitsinformation about the timing of the received RF signals to NRS 112 vianetwork 110. In this embodiment, NRS then calculates the location ofsecond base station 106 and transmits the location back to second basestation 106 over network 110. Additionally, the physical location of NRS112 may be determined based on the timing of the RF signals 124 receivedat NRS 112.

Once the respective locations of second base station 106 and NRS 112 aredetermined, one or more of RF signals 124 are selected which is thenused to calculate a clock offset value to synchronize local clock 116with reference clock 114. Further description of synchronizing localclock 116 and calculating the clock offset value is provided in moredetail below.

FIG. 2 is a functional block diagram illustrating a base station 206, inaccordance with an embodiment of the invention. Base station 206represents one possible implementation of second base station 106, shownin FIG. 1. The illustrated embodiment of base station 206 includes asystem bus 202, a radio receiver 204, a radio antenna 208, a processingdevice 210, a Random Access Memory (RAM) 212, a non-volatile storage(NVS) 214, a local clock 216, a wireless interface 218, a wirelessantenna 220, and a network interface 222.

In the illustrated embodiment, wireless interface 218 is configured tosend and receive wireless communications to and from a mobile device(e.g., mobile device 102 of FIG. 1) via wireless antenna 220. In oneembodiment, wireless interface 218 is configured to communicate with amobile device in accordance with a CDMA standard, such as CDMA2000.Alternatively, wireless interface 218 may be configured to communicatewith a mobile device in accordance with a TDMA standard, a GSM standard,a UMTS standard, or a WiMAX standard.

Local clock 216 is coupled to provide wireless interface 218 with alocal clock signal 224. Wireless interface 218 uses local clock signal224 as the basis for timing its wireless communications with a mobiledevice. For example, if wireless interface 218 is configured tocommunicate in accordance with a TDMA standard, wireless interface 218may communicate with multiple mobile devices, each during a respectivetime slot, where the timing of each time slot is determined in responseto local clock signal 224.

Radio receiver 204 is configured to receive RF signals 124 via radioantenna 208. Radio receiver 204 may be configured to receive RF signalsincluding, but not limited to, broadcast television signals (e.g., NTSC,ASTC DVT, DVB, and ISDB) or broadcast radio signals (e.g., FM, AM, andDAB). In one embodiment, radio receiver 204 is configured to receive acombination of the aforementioned RF signals. For example, radioreceiver 204 may receive one or more NTSC television signals and one ormore ASTC DVT television signals. In another example, radio receiver 204may receive one or more broadcast television signals and one or morebroadcast radio signals.

Upon receiving RF signals, radio receiver 204 determines timinginformation pertaining to when each RF signal was received at basestation 206. Radio receiver 204 then transmits this timing informationto processing device 210 and/or to network interface 222 via system bus202. In one embodiment, RAM 212 or NVS 214 includes the location of theRF transmitters that transmitted the received RF signals. Using theknown locations of the RF transmitters, processing device 210 calculatesthe location of base station 206 in response to the timing informationreceived from radio receiver 204. In another embodiment, the location ofbase station 206 is calculated by a device separate from base station206, such as NRS 112 of FIG. 1. In this embodiment, network interface222 transmits the timing information to a network reference server(e.g., NRS 112 of FIG. 1) via network 110. In response, networkinterface 222 receives the location of base station 206 from the networkreference server. Network interface 222 may then transmit the locationinformation to processing device 210 via system bus 202 for furtherprocessing.

As mentioned above, synchronization among base stations in acommunications network is desirable to maintain uninterrupted calls as amobile device is handed off from one base station to another. However,in one embodiment, base station 206 may not include a direct connection(e.g., microwave, ISDN, or fiber optic) to a master reference clock suchas reference clock 114, shown in FIG. 1. Instead, base station 206includes radio receiver 204, processing device 210, and networkinterface 222 to aide in the synchronization of local clock 216 with amaster reference clock of the communications network. For example, radioreceiver 204 may receive one or more RF signals and then transmit timinginformation about when those RF signals were received to networkinterface 222 via system bus 202. Network interface 222 then transmitsthis timing information to a network reference server (e.g., NRS 112 ofFIG. 1). The network reference server then calculates a clock offsetvalue and transmits it back to base station 206. Network interface 222receives the calculated clock offset value via network 110 and transmitsthe clock offset value to wireless interface 218 via system bus 202.Wireless interface 218 then adjusts local clock 216 in response to theclock offset value such that local clock 216 is substantiallysynchronized with the master reference clock of the communicationsnetwork.

In one embodiment, network interface 222 includes a discrete multi-tone(DMT) communication device, such as a Digital Subscriber Line (DSL)modem. The DMT device may be connected to network 110 via an AsymmetricDSL (ADSL) line, a Very high bit-rate DSL (VDSL) line, a Symmetric DSL(SDSL) line, a Rate-adaptive DSL (RADSL) line, or the like. The DMTsystem transmits and receives an information bit stream from network110. The information bit stream is typically converted into a sequenceof data symbols having a number of tones. Each tone may be a group ofone or more frequencies defined by a center frequency and a setbandwidth. The tones are also commonly referred to as sub-carriers orsub-channels. Each tone acts as a separate communication channel tocarry information between base station 206 and a remote transceiverdevice (e.g., in network reference server 312). In one embodiment, theDMT communication device is a DSL modem that may be connected tocomputer network 110 via a telephone line of the public switchedtelephone network (PSTN). In one embodiment, the DMT system divides theavailable bandwidth of the telephone line into a plurality ofapproximately 4 kHz wide channels. The DMT system then monitorscommunications on each channel and shifts data signals from poor qualitychannels to better quality channels. In another embodiment, networkinterface 222 includes a cable modem or a dial-up modem. In yet anotherembodiment, network interface 222 includes a Wi-Fi device forcommunicating over network 110 via a wireless local area network (notshown).

In one embodiment, wireless interface 218 synchronizes local clocksignal 224 with a master reference clock (e.g., reference clock 114 ofFIG. 1) by delaying local clock signal 224 for an amount of timeindicated by the clock offset value. In another embodiment, wirelessinterface 218 synchronizes local clock signal 224 with a masterreference clock by reducing a phase difference between local clocksignal 224 and the master reference clock.

In one embodiment, processing device 210 includes a microprocessor, amicrocontroller, or the like. Processing device 210 is coupled to NVS214, which may be used to store firmware (e.g., control algorithmsexecutable by processing device 210 to implement any of the processesdescribed herein).

FIG. 3 is a functional block diagram illustrating a network referenceserver (NRS) 312, in accordance with an embodiment of the invention. NRS312 represents one possible implementation of NRS 112 of FIG. 1. Theillustrated embodiment of NRS 312 includes a system bus 302, a radioreceiver 304, a radio antenna 308, a processing device 310, a localclock 316, a clock interface 318, a network interface 322, a RandomAccess Memory (RAM) 324, and a non-volatile storage (NVS) 326.

Clock interface 318 is coupled to receive reference clock signal 118from reference clock 114. In one embodiment, reference clock signal 118is a master reference clock signal of a communications system to whichvarious base stations are synchronized. In one embodiment, clockinterface 318 receives reference clock signal 118 via an interfacewithout substantial delay or via an interface where the delay isreliably predictable. For example, clock interface 318 may include anISDN (e.g., T1) modem, a point to point microwave interface, or a fiberOptic (e.g., OC3) connection.

Local clock 316 is coupled to clock interface 318. In one embodiment,clock interface 318 is configured to periodically synchronize localclock 316 with reference clock signal 118. Local clock 316 is alsocoupled to output a local clock signal 314 to processing device 310 viasystem bus 302. Although FIG. 3 shows NRS 312 as including local clock316, it is recognized that local clock 316 may be omitted and clockinterface 318 coupled directly to system bus 302.

Radio receiver 304 is configured to receive RF signals via radio antenna308. Radio receiver 304 may be configured to receive RF signalsincluding, but not limited to, broadcast television signals (e.g., NTSC,ASTC DVT, DVB, and ISDB) or broadcast radio signals (e.g., FM, AM, andDAB). In one embodiment, radio receiver 304 is configured to receive acombination of the aforementioned RF signals. For example, radioreceiver 304 may receive one or more NTSC television signals and one ormore ASTC DVT television signals. In another example, radio receiver 304may receive one or more broadcast television signals and one or morebroadcast radio signals.

Upon receiving RF signals, radio receiver 304 determines timinginformation pertaining to when each RF signal was received at NRS 312.Radio receiver 304 then transmits this timing information to processingdevice 310. In one embodiment, RAM 326 or NVS 324 includes the locationsof a plurality of RF transmitters (e.g., RF transmitters 108 of FIG. 1).In another embodiment, the locations of the RF transmitters are storedin a database (not shown) located locally in base station 206 orelsewhere in network 110. Using the known locations of the RFtransmitters, processing device 310 calculates the location of NRS 312in response to the timing information received from radio receiver 304.

In one embodiment, the location of a base station, such as base stations106 and 206, are determined by NRS 312. In this embodiment, networkinterface 322 receives timing information pertaining to when the RFsignals were received by a base station. In response, network interface322 transmits this timing information to processing device 310 viasystem bus 302. Using the known locations of the RF transmitters,processing device 310 calculates the location of the base station inresponse to the timing information received from the base station. Thelocation of the base station may then be transmitted back to the basestation via network interface 322 and network 110.

In one embodiment, NRS 312 serves as a locator of a plurality of basestations. For example, several base stations geographically spaced apartmay transmit RF timing information to the NRS 312 via network 110. NRS312 may then determine the location of each base station and transmitthe location of each back to a respective base station.

NRS 312 may also serve as a central access point for synchronizationinformation for several base stations. For example, in one embodiment, aplurality of base stations that do not have direct access (e.g.,microwave, ISDN, or fiber optic) to a master reference clock of acommunications system may transmit timing information pertaining to whenRF signals were received at the base stations. In response, NRS 312 maycalculate a time delay or clock offset value for each base station andtransmit it back to each respective base station so that each maysynchronize its local clock with the reference clock 114.

In one embodiment, processing device 310 includes a microprocessor, amicrocontroller, or the like. Processing device 310 is coupled to NVS314, which may be used to store firmware (e.g., control algorithmsexecutable by processing device 310 to implement any of the processesdescribed herein).

FIG. 4 is a flow chart illustrating a process 400 for synchronizing abase station to a wireless communications system, in accordance with anembodiment of the invention. Process 400 is described with reference toFIGS. 1-4. The order in which some or all of the process blocks appearin process 400 should not be deemed limiting. Rather, one of ordinaryskill in the art having the benefit of the present disclosure willunderstand that some of the process blocks may be executed in a varietyof orders not illustrated.

In one embodiment, mobile device 102 communicates with first basestation 104. Mobile device 102 may either have initiated acommunications link with first base station 104 (e.g., placed a call) orfirst base station 104 may have initiated a communications link withmobile device (e.g., receiving a call). Alternatively, mobile device 102may have been handed off from another base station as mobile device 102traveled into coverage area 126. As mobile device 102 travels intooverlapped coverage area 130 a decision is made whether to hand offmobile device from first base station 104 to second base station 106. Inone embodiment, the decision whether to hand off may be made, in part,based on relative signal strengths of the respective coverage areas 126and 128. For example, second base station 106 may provide a coveragearea 128 that extends primarily inside of a building, while first basestation 104 provides a coverage area 126 that extends primarilyoutdoors. In this example, as mobile device 102 travels from outdoors toinside of the building the wireless signal strength provided by firstbase station 104 may be greatly reduced, while wireless signal strengthprovided by second base station 106 is stronger. In order to improvecall quality and reliability a mobile device 102 should be handed offfrom first base station 104 to second base station 106.

In one embodiment, the decision to hand off may be made based on aneconomic incentive. For example, as described above second base station106 may be a home base station for extending cellular coverage ofcommunications network 100 to inside of a building. In this example,voice and data information may be packetized and transmitted to thecommunications network via computer network 110. As may be the case,transmitting voice and data over computer network 110 rather than usingbandwidth of first base station 104 may be cheaper for a subscriber ofcellular services. In this regards, it may always be decided to handoffto second base station 106 if mobile device 102 is within coverage area128.

As mentioned above, synchronization among base stations in acommunications network is desirable to maintain uninterrupted calls as amobile device is handed off from one base station to another.Furthermore, synchronization among base stations may be desirable toreduce interference among adjacent or near base stations. Process 400provides one possible implementation of synchronizing a base station,where in a process block 406, the process of synchronizing local clock116 to reference clock 114 begins. Process 400 of synchronization mayoccur periodically as suited for a particular purpose of communicationssystem 100.

FIG. 4 includes boxes labeled “Second Base Station” and “NRS”. Processblocks 406 through 416 are shown as included in one of these two blocksto indicate a respective device of communications system 100 thatperforms those functions in the illustrated embodiment. For example,process block 406 indicates that RF signals 124 are received at secondbase station 106. Process block 406 is shown as included in the boxlabeled “Second base Station”. Thus, in this embodiment, second basestation 106 is responsible for performing process block 406. However, itis appreciated that various process blocks described herein may beperformed at either second base station 106 or at network referenceserver (NRS) 112, depending on the particular implementation ofcommunications system 100. For example, after RF signals 124 arereceived at second base station 106, process 400 continues to a processblock 408, where the location of second base station 106 is determined.As discussed above, the location of second base station 106 may bedetermined by either second base station 106 or by NRS 112. If thelocation of second base station 106 is determined by second base station106, itself, the known locations of RF transmitters 108 and the timethat RF signals were received by second base station 106 are used tocalculate the location of base station 106. In one embodiment, thelocation of second base station 106 is calculated to withinapproximately one foot. If the location of second base station 106 isdetermined by NRS 112, the time that RF signals were received by secondbase station 106 is transmitted to NRS 112 via network 110, where asimilar calculation is made by NRS 112. NRS 112 may then transmitlocation information back to second base station 106 via network 110.

In one embodiment, determining the location of second base station 106includes calculating a distance from at least one of the RF transmitters108 to second base station 106. In another embodiment, the elevation ofsecond base station 106 and at least one of the RF transmitters 108 isdetermined to more accurately determine a distance between the two. Forexample, after the location of second base station 106 is determined,the location may be cross-referenced with a topological map to determinean elevation of second base station 106.

In process blocks 410 and 412, NRS 112 receives RF signals 124 anddetermines a location of NRS 112 in a similar manner as is used todetermine the location of second base station 106, described above. Forpurposes of determining the location of NRS 112, it is recognized thatthe set of RF signals 124 that were used to determine the location ofsecond base station 106 need not be the same set of RF signals used todetermine the location of NRS 112. Also, in one embodiment, determiningthe location of NRS 112 may include calculating a distance from at leastone of the RF transmitters 108 to the NRS 112. In another embodiment,the elevation of NRS 112 and at least one of the RF transmitters 108 isdetermined to more accurately determine a distance between the two.

In a process block 414, a clock offset value is determined. In oneembodiment, clock offset value represents an amount of time to delaylocal clock 116 such that local clock 116 and reference clock 114 aresubstantially synchronized. In another embodiment, clock offset valuerepresents a phase difference between local clock signal 224 (shown inFIG. 2) and reference clock signal 118 (shown in FIG. 3).

After the clock offset value is calculated, second base station 106synchronizes local clock 116 with reference clock 114 in a process block416. In one embodiment, second base station 106 synchronizes local clock116 by reducing a phase difference between local clock signal 224 andreference clock signal 118 to below a phase difference threshold. Thephase difference threshold may be a predetermined value set in thesecond base station 106 or it may be a dynamic value that changesdepending on a desired system performance.

After local clock signal 224 is synchronized with reference clock signal118, mobile device 102 may be handed off to second base station 106 witha reduced risk of dropping the connection between mobile device 102 andcommunications network 100. In one embodiment, wireless interface 218(shown in FIG. 2) negotiates with first base station 104 to complete thehand off of mobile device 102. In one embodiment, mobile device 102performs a hard hand off, where the communication link between firstbase station 104 and mobile device 102 is first broken and then acommunication link between mobile device 102 and second base station 106is established. In another embodiment, mobile device 102 performs a softhand off, where the communication link between first base station 104and mobile device 102 is maintained until a communication link betweenmobile device 102 and second base station 106 is established. After thehand off is complete, mobile device 102 communicates with second basestation 106, continuing its previously established call.

FIG. 5A is a flow chart illustrating a process 414A for determining aclock offset value, in accordance with an embodiment of the invention.Process 414A represents one possible implementation of process block 414of FIG. 4, where clock offset value is calculated at network referenceserver 112. FIG. 6 is a timing diagram 600 illustrating radio frequency(RF) signal 124 and a reference clock signal 118, in accordance with anembodiment of the invention. Process 414A is described with reference toFIGS. 5A and 6.

As shown in FIG. 6, RF signal 124 is transmitted from an RF transmitter108 at a time T0. Pulse 610 may represent one or more time stampsincluded in RF signal 124. For example, pulse 610 may include apresentation time stamp (PTS) or a decode time stamp (DTS) of an ASTCDTV broadcast television signal.

After RF signal 124 is transmitted from RF transmitter 108, it isreceived at NRS 112 at a time T1. Time T1 is later than time T0 due to apropagation delay of RF signal 124, shown as time delay TD1 in FIG. 6.In a process block 502, NRS 112 determines the time T1 at which RFsignal 124 was received at NRS 112. Since the locations of NRS 112 andRF transmitter 108 are known and the speed of light is constant, time T0can be calculated by subtracting an expected time delay TD1 from time T1(e.g., process block 504).

Using the calculated time T0, a time delay TD3 can now be calculated ina process block 506. As shown in FIG. 6, time delay TD3 represents anamount of time between time T0 and a rising edge of reference clocksignal 118. In another embodiment, time delay TD3 may be measured fromtime T0 to a falling edge of reference clock signal 118. In yet anotherembodiment, time delay TD3 may be measured from time T0 to apredetermined fixed point of reference clock signal 118.

The same RF signal 124 is also received at second base station 106 at atime T2 due to another propagation delay, shown as time delay TD2. In aprocess block 508, time T2 is determined at second base station 106.Although FIG. 6 shows time T2 as later than time T1, it is recognizedthat time T2 can be sooner, the same, or later than time T1, dependingon the relative distance of second base station 106 from RF transmitter108. In one embodiment, time T0 can also be calculated by subtracting anexpected time delay TD2 from time T2 since the location of second basestation 106 is also known.

In a process block 510, time T2 is transmitted to NRS 112 via network110. Now that time T0 is calculated and time delay TD3 is alsocalculated, a clock offset value can be calculated (e.g., process block512) so that local clock 116 can be synchronized to reference clock 114.In a process block 514, the calculated clock offset value is transmittedfrom NRS 112 to second base station 106 via network 110, where process400 continues to process block 416 to synchronize local clock 116 withreference clock 114 in response to the received clock offset value.

FIG. 5B is a flow chart illustrating another process 414B fordetermining a clock offset value, in accordance with an embodiment ofthe invention. Process 414B represents another possible implementationof process block 414 of FIG. 4, where clock offset value is calculatedat second base station 106. Process 414B is described with reference toFIGS. 5B and 6.

In a process block 520, NRS 112 determines the time T1 at which RFsignal 124 was received at NRS 112. Since the locations of NRS 112 andRF transmitter 108 are known and the speed of light is constant, time T0can be calculated by subtracting an expected time delay TD1 from time T1(e.g., process block 522).

Using the calculated time T0, a time delay TD3 can now be calculated ina process block 524. In a process block 526, time delay TD3 istransmitted to second base station 106 via network 110.

The same RF signal 124 is also received at second base station 106 at atime T2 due to another propagation delay, shown as time delay TD2. In aprocess block 528, time T2 is determined at second base station 106. Ina process block 530, a clock offset value is calculated based in part onthe received time delay TD3. Process 400 then continues to process block416 to synchronize local clock 116 with reference clock 114 in responseto the clock offset value calculated at second base station 106.

Referring again to FIG. 6, a demonstrative example is given. In thisexample RF transmitter 108 is located a distance of 1 mile from NRS 112.The expected time delay (i.e., TD1) between when RF transmitter 108transmits RF signal 124 and when NRS 112 receives RF signal 124 is 5280nanoseconds. In this example, if second base station 106 is 2 miles awayfrom RF transmitter 108, time delay TD2 will be 10560 nanoseconds. TD3is then measured. In this example, TD3 is measured as 1000 nanoseconds(measured from time T0 to a rising edge T3 of reference clock signal118). Given the time T0, T2, and the time delay TD3, a clock offsetvalue can be calculated to synchronize the local clock signal at thesecond base station with reference clock signal 118. For example, giventhe above calculations, second base station 106 now knows when T3 occursand may use time T2 as a reference to adjust local clock 116 to matchreference clock signal 118

Another approach to provide synchronization between base stations is foreach base station to utilize a highly accurate oscillator, such as anOven Controlled Crystal Oscillator (OCXO). The OCXO can be set when thebase station is first powered on and then can run free without anyfeedback as its drift is small enough over time and through temperaturechanges so as to stay synchronized. However, OCXOs are much moreexpensive than standard crystal oscillators.

Still, another approach to synchronize a base station is to use aTemperature Compensated Voltage Controlled Crystal Oscillator (TCVCXO)with feedback provided using an IP network and the IEEE1588 standard.However, this approach requires a network delay that is consistent andguaranteed, such as with an ISDN (e.g., T1), a point to point microwave,or a fiber Optic (e.g., OC3) connection.

Yet another approach for synchronization is for the base stations toutilize Global Positioning System (GPS) signals to synchronize theirlocal clocks. Each base station may include a GPS receiver to receivepositioning information via a satellite. Such base stations may use thevery accurate GPS clock signal that is transmitted by the satellitealong with the positioning information to synchronize the base station'slocal clock with a master reference clock provided by the communicationsnetwork. Thus, all base stations in the communications network couldsynchronize their local clocks with the system-wide master referenceclock by utilizing the GPS signal. In this way each base station's clockis adjusted to the master reference clock so as to precisely aligntimeslot information with all the other cells. With their local clockssynchronized, base stations may be able to provide seamless handoffs ofmobile devices from one base station to another.

However, the OCXO, TCVCXO, and GPS approaches for synchronizing a basestation with a communications network may not always be feasible with ahome base station. For example, as stated above, OCXOs may be tooexpensive to implement into a home base station. Also, a home or officethat utilizes a broadband connection may include network delays that areneither consistent nor guaranteed (e.g., cable or DSL). Also, GPSsignals received inside of a building may not be strong enough orcoherent enough to be used by the home base station to synchronizeitself with other base stations of the wireless communications network.As a result of these limitations of the OCXO, TCVCXO, and GPSapproaches, a handoff of a mobile device between an existing basestation and a home base station may fail or be significantly delayed andmay, for example, result in the dropping of an active call.

The processes explained above are described in terms of computersoftware and hardware. The techniques described may constitutemachine-executable instructions embodied within a machine (e.g.,computer) readable medium, that when executed by a machine will causethe machine to perform the operations described. Additionally, theprocesses may be embodied within hardware, such as an applicationspecific integrated circuit (“ASIC”) or the like.

A machine-accessible medium includes any mechanism that provides (i.e.,stores and/or transmits) information in a form accessible by a machine(e.g., a computer, network device, personal digital assistant,manufacturing tool, any device with a set of one or more processors,etc.). For example, a machine-accessible medium includesrecordable/non-recordable media (e.g., read only memory (ROM), randomaccess memory (RAM), magnetic disk storage media, optical storage media,flash memory devices, etc.).

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification. Rather, the scope of the invention is tobe determined entirely by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

1. A server, comprising: a radio receiver coupled to receive a radio frequency (RF) signal at a time T1; a clock interface to receive a reference clock signal; a network interface to couple the server to a base station via a computer network, the network interface configured to receive a time T2 from the base station, the time T2 representative of a time that the RF signal was received at the base station; and a processing device coupled to the radio receiver, the clock interface and the network interface, wherein the processing device is configured to use the time T1 to determine a time T0 when the RF signal was transmitted and a location of the base station, and wherein the processing device is configured to calculate a clock offset value based on when the RF signal was transmitted, the location of the base station, the time T2, and a reference clock signal, the clock offset value representative of a time to delay a local clock signal at the base station to synchronize the local clock signal at the base station with the reference clock signal.
 2. The server of claim 1, wherein the network interface is configured to transmit the calculated clock offset value to the base station.
 3. The server of claim 1, wherein the processing device is further configured to calculate a time T0 in response to a time T1 at which the RF signal was received by the radio receiver, wherein time T0 represents a time when the RF signal was transmitted by an RF transmitter.
 4. The server of claim 1, wherein the RF signal is one of a plurality of RF signals and the radio receiver is coupled to receive the plurality of RF signals, wherein the processing device is further configured to calculate a distance between the server and an RF transmitter in response to the received plurality of RF signals, wherein the RF transmitter transmitted at least one of the plurality of RF signals.
 5. The server of claim 1, wherein the network interface is configured to receive data from the base station via the computer network, the data representative of a plurality of RF signals received at the base station, wherein the processing device is further configured to calculate the location of the base station in response to the received data.
 6. The server of claim 1, wherein the RF signal comprises a broadcast television signal that includes periodic timestamp information.
 7. The server of claim 1, wherein the clock interface receives the reference clock signal from a master reference clock via a fiber optic link.
 8. The server of claim 1, wherein the clock interface receives the reference clock signal from a master reference clock via an ISDN link.
 9. A method, comprising: receiving, by a radio receiver of a server at a time T1, a radio frequency (RF) signal; receiving, by a clock interface of the server, a reference clock signal; and receiving, by a network interface of the server, a time T2 from a base station coupled to the server via a computer network, the time T2 representative of a time that the RF signal was received at the base station; calculating, by a processing device of the server, a location of the base station in response to the time T2; and calculating, by the processing device of the server, a clock offset value in response to the received RF signal, the RF signal received at the base station, and the reference clock signal, wherein calculating the clock offset value comprises using the time T1 to calculate a time T0 when the RF signal was transmitted by an RF transmitter, and wherein the clock offset value is based on the time T0, the location of the base station, the time T2, and the reference clock signal, the clock offset value used to synchronize a local clock signal at the base station with the reference clock signal.
 10. The method of claim 9, further comprising: transmitting, by the network interface of the server, the calculated clock offset value to the base station.
 11. The method of claim 9, wherein receiving the RF signal comprises receiving a first plurality of RF signals, the method further comprising: calculating a location of the server in response to the received plurality of RF signals.
 12. The method of claim 9, wherein calculating the clock offset value further comprises: determining a time T1 when the RF signal was received at the server; calculating the time T0 in response to the time T1 and in response to a distance between the RF transmitter and the server; and calculating the clock offset value in response to time T2 and a delay between time T0 and the reference clock signal.
 13. The method of claim 9, wherein the RF signal comprises a broadcast television signal.
 14. The method of claim 9, wherein the clock interface receives the reference clock signal from a master reference clock. 